1. Field of the Invention
The present invention relates to a zoom lens for projection and a projection-type display device on which the zoom lens for projection is mounted.
2. Description of the Related Art
In recent years, projection-type display devices, such as transmission-type or reflection-type liquid crystal display devices and DMD (digital micromirror device) display devices, which use various kinds of light bulbs and have relatively long back focus, were widely used. Further, the performance of such devices has been becoming higher.
The projection-type display device uses, as a projection lens, a zoom lens, which can change the size of an image formed on a screen, in many cases. Recently, there is an increasing demand for a zoom lens that can greatly change the size of the image, in other words, a zoom lens having a higher zoom ratio.
Further, a lens shift function and a widening function of the zoom lens became required. The lens shift function shifts the center of a projected image with respect to the projection-type display device, for example, to the upper side. The widening function can project a large image at a position close to the projection-type display device. Therefore, zoom lenses having wider angles (wider angles of view) are needed.
Further, an optical system or the like using a plurality of light bulbs needs space for inserting a prism that combines, together, light beams of various colors output from the plurality of light bulbs respectively. Therefore, long back focus is required.
Conventionally, this kind of projection-type zoom lens disclosed, for example, in Japanese Unexamined Patent Publication No. 2005-106948 (Patent Document 1) was known. In the zoom lens disclosed in Patent Document 1, the zoom ratio is greater than or equal to 1.5 times, which is relatively high. However, with respect to the angle of view, the zoom lens disclosed in Patent Document 1 does not satisfy the need for widening the angle of view in recent years.
Further, in recent years, the sizes of light bulbs became smaller, and the resolution of pixels became higher. Therefore, lateral chromatic aberration of a zoom lens for projection needs to be reduced further. However, in the zoom lens disclosed in Patent Document 1, the lateral chromatic aberration was not reduced sufficiently. Especially, the lateral chromatic aberration is an important factor that determines the image quality. If the lateral chromatic aberration is larger than or equal to half of the pixel of the light bulb, the quality of the projected image deteriorates extremely, and the quality of the image becomes intolerable for practical use in some cases.
As a method for reducing the lateral chromatic aberration, a method using a low-dispersion glass material is known.
When Abbe number νd of the low-dispersion glass material exceeds 65, the change (dn/dt) in the refractive index of the low-dispersion glass material with respect to a change in temperature is negative in most cases (the sign of the change is minus). When a low-dispersion glass material having higher Abbe number νd (for example, νd≧80) is used, the value of dn/dt is a negative value having a large absolute value.
Therefore, when the low-dispersion glass material is used for a lens having a positive refractive power, the focused position of the lens shifts toward the rear side (reduction side) as temperature increases, and the magnitude of the shift is greater, as the Abbe number of the material is higher. Further, a retrofocus-type lens, in which a lens group having a negative refractive power is located on the front side of the lens, is used in some cases, because it is relatively easy to increase the angle of view and to maintain long back focus. In the retrofocus-type lens, it is necessary to arrange many positive lenses on the reduction side of the pupil position thereof. Therefore, it is possible to correct the lateral chromatic aberration by using a low-dispersion glass material for the positive lenses.
However, when the positive lenses are made of low-dispersion glass material, as described above, especially if the Abbe number of the material is greater than or equal to 65, the change (dn/dt) in the refractive index with respect to the change in temperature of the material becomes large. Therefore, out-of-focus condition and deterioration in the optical performance, caused by the change in temperature, become noticeable, and even intolerable for actual use in some cases.